At least one opening or vent is commonly provided in a tank (such as a reactor, a processing device, a storage tank, a gas holder or other vessel) for the discharge, or the release, of an undesirably large pressure differential relative to the adjacent environment. To regulate use or operation of the tank, avoid material loss from the tank, prevent damage to the tank, and/or minimize or even prevent environmental contamination with material from the tank, such a vent is commonly provided with a vent closure that can open at a preset pressure differential.
While various vent closure structures are known, one particularly effective structure utilizes a combination of pallet and diaphragm. The pallet in normal valve operation extends across and closes the vent mouth while the associated diaphragm seats about and seals the vent mouth. The pallet and diaphragm combination can be considered to comprise or to be included in a vent closure cover subassembly.
One commonly used type of vent closure structure that incorporates a pallet and diaphragm combination is a gauge hatch. Characteristically, a conventional gauge hatch has a radially extending pivot arm that is associated with an exterior central portion of the closure cover subassembly and that is pivotably associated with an exterior cover-adjacent hitching or pivot post. Thus, this pivot arm permits the normally closed cover subassembly to be pivoted to a vent open position where the cover assembly and pivot arm do not interfere with tank interior access for tank measurements that are made through a vent by an operator. The cover subassembly in a gauge hatch thus can be assembled so as to be either opened manually by the operator, or opened automatically if the associated tank reaches a set pressure where pressure in the tank needs to be relieved.
Various means are known for regulating the opening or set pressure in a gauge hatch-equipped vent where, at vent opening, the pallet and diaphragm combination separate from the valve mouth. For example, one particularly effective pressure regulating means comprises a weight that is inherently associated with, or that is effectively loaded onto the upper outside surface of the cover. Thus, in such an arrangement, the weight-loaded pallet and diaphragm combination of a cover separates from its associated seated configuration at a preset tank pressure (which can be super-atmospheric), thereby to achieve vent closure opening.
Some gauge hatches are provided with retainer means for optionally holding the closure cover subassembly in a "bolted down" or equivalent vent closed configuration. The use of the retainer means can be an operator-selected option. Such a gauge hatch is typically utilized for association with a tank that is also equipped with a vent valve which automatically opens when the associated tank reaches a set pressure.
One problem with such a pallet and diaphragm-type gauge hatch is that a small volume of fugitive vapor emissions (typically parts per million) may occur between the diaphragm and its seat before either the set opening pressure of the gauge hatch cover is reached or the gauge hatch cover is manually opened by an operator.
The present invention overcomes this fugitive emissions problem for pallet and diaphragm type gauge hatch vent closures by incorporating into the gauge hatch structure a cooperating liquid seal. The liquid seal can function to virtually eliminate the escape of all fugitive vapor emissions prior to the opening of the pallet and diaphragm combination either at a set opening pressure or at manual opening.